Fabrication of most micro- and nano-devices including semiconductors, photonic and optoelectronic devices, microelectromechanical systems/nanoelectromechanical systems (MEMS/NEMS), electronic displays (such as Liquid Crystal Displays (LCDs)), etc. requires the deposition of many thin films. Several deposition options exist in the industry today. Deposition in the liquid phase is typically carried out by processes, such as spin-coating, which is often used as a precursor to subsequent reactions that solidify the liquid to obtain the desired thin film. In the vapor phase, the most commonly used technique is Chemical Vapor Deposition (CVD). In a typical CVD process, the substrate is exposed to precursors in the gaseous phase that react or decompose to form the desired film on the surface of the substrate. There are several types of CVD processes. Depending upon the pressure used, they can be classified as Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD) or Ultrahigh Vacuum CVD (UHVCVD). Low pressures tend to reduce unwanted reactions and improve film thickness uniformity. Plasma based methods to enhance the chemical reactions, such as Plasma Enhanced CVD (PECVD) and Remote PECVD, are also used in the deposition of thin films in the semiconductor industry to lower deposition temperatures and protect the substrate from high-temperature effects. A technique called Atomic Layer Deposition (ALD) is also frequently used to produce conformal monolayers of one or different materials. Physical Vapor Deposition (PVD) methods too are important thin film deposition techniques. As the name suggests, they do not rely on chemical reactions, but deposit condensed forms of a vaporized material onto the substrate in a vacuum environment. Evaporative deposition and sputtering are two common examples of PVD. The former heats the material to be deposited to a high vapor pressure, while the latter utilizes a plasma discharge to bombard the substrate surface with atoms of the material to be deposited.
All the processes discussed above deposit thin films in a manner where the amount of material deposited per unit area is substantially the same. The ability to tailor materials to form intentionally non-uniform films is not typically possible for these processes, or requires frequent changes in hardware or tooling to accommodate variations in substrate geometry and desired film thickness profile. Also, processes, such as spin-coating, involve significant material wastage, while vacuum processes can be expensive due to the need to pump down chambers where processing is performed.
With the need for more sustainable processes, inkjetting has also become an attractive technique for material deposition as well as inexpensive patterning due to its direct write, “maskless” nature. However, due to the presence of a substantial vapor-liquid interface in the dispensed drops, evaporation and gradients in surface tension can cause local film thickness non-uniformity leading to the infamous “coffee-ring effect.” Moreover, film thickness uniformity is also influenced strongly by the volume of the individual drops, the surface properties of both, the dispensed fluid as well as the substrate, and the spacing between consecutive drops, or the drop pitch, which needs to be low enough to allow the drops to spread and merge. Hence, in spite of having remarkably low material consumption, the above factors make process control for inkjet-based deposition of large area nanoscale thickness films challenging.
Flow coating has been developed at National Institute of Standards and Technology (NIST) as a velocity-gradient knife-edge coating process. A drop of the polymer solution is deposited on the substrate which is moved at constant acceleration. The competition between friction drag as a result of the velocity gradient action of the substrate and the capillary forces due to the stationary knife-edge placed ˜200 μm above the substrate during the substrate motion creates a thickness gradient of the film. Subsequent evaporation leads to the realization of sub-micron thickness films. Thin polystyrene films with range even in the sub-200 nm regime have been demonstrated using this apparatus, but it is unclear whether films in non-monotonic profiles can be obtained using the same.
A variation of electrochemical deposition has also been used, in which varying thickness poly-electrolyte films have been deposited using spatially tunable electric field gradients. In addition, variable salt etching of polyelectrolyte films where the amount of material removed is controlled spatially to realize thin film thickness gradients has also been demonstrated. However, such techniques do not have the film thickness range and resolution required to be applicable for a broad spectrum of areas.
The deposition of ultra-thin films with functional gradients is an active area of research in the biomedical domain related to studying various factors involved in tissue engineering. To this end, biomimetic films have been fabricated by a layer-by-layer (LBL) assembly process, where it is possible to impart functional gradients at a molecular level and higher to screen events, such as protein adsorption and cell adhesion. The LBL techniques mainly proceed through a combination of various surface interactions, including electrostatic forces, van-der-Waals forces, hydrogen bonds, etc. Grafting of polymer molecules on functionalized substrates with temperature gradients also results in thickness gradients.
In addition to the above mentioned methods, vapor-based techniques are also available, primarily for depositing inorganic films with varying thicknesses. These techniques mostly employ a motion-controlled mask to generate the required thickness profiles, or use a discretized shower head with control over each shower unit. Such methods have limited film thickness variations that can be achieved and often require a change in hardware to generate a variety of profiles, thus constraining their versatility across various applications.
Hence, the currently used techniques for film deposition do not have the film thickness range and resolution required to be applicable for a broad spectrum of areas and have limited film thickness variations that can be achieved thereby requiring a change in hardware to generate a variety of profiles, thus constraining their versatility across various applications.